A CDMA cellular network is a digital spread spectrum communications system. The CDMA network includes several base stations each providing digital service to mobile units located in different geographical regions. Communication between a mobile unit and a base station in an IS-95A-based CDMA network occurs on reverse and forward CDMA channels. The reverse CDMA channel is a mobile unit-to-base station direction of communication that carries traffic and signaling information. The forward CDMA channel is a base station-to-mobile unit direction of communication that carries pilot, sync, and paging signals in addition to traffic signals.
The reverse CDMA channel includes access channels and reverse traffic channels. The access channels are used by the mobile unit to initiate communication with a base station, and to respond to paging channel requests.
The forward CDMA channel consists of a pilot channel, a sync channel, up to seven paging channels, and up to sixty-three forward traffic channels. Each of these channels is orthogonally spread by an appropriate Walsh function and then spread by the quadrature pair of PN sequences (I and Q) at a fixed rate of 1.2288 million chips per second.
The base station of a sector continuously transmits on the pilot channel of each active forward CDMA channel. A mobile unit operating within the coverage area of the base station uses this continuous transmission for synchronization. The network assigns each base station of the cell site a specific time (or phase) offset of the pilot PN sequence to identify a forward CDMA channel. A given base station uses the same pilot PN sequence offset, or simply PN offset, on all CDMA frequency assignments. For example, all traffic, sync, and paging channels transmitted from a single base station share the same PN offset. An offset index (0 through 511 inclusive) identifies distinct pilot channels. This offset index specifies the offset value from the zero offset pilot PN sequence. Each offset index increment represents the interval between pilot channels in increments of 64 chips (i.e., 52.08 ms).
An active mobile unit maintains four sets of pilot channels when communicating with a base station of a CDMA sector: the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set consists of all the pilot channels that the mobile unit is currently using for demodulation. The Candidate Set contains all of the pilot channels that are not currently in the Active Set but have sufficient signal strength to be considered for soft or softer handoff. The Neighbor Set contains pilot channels that are not currently in the Active or Candidate Set, but may become eligible for handoff (e.g., pilot channels of nearby sites). The Remaining Set is the set of all possible assigned pilot channels in the CDMA network on the same carrier frequency, excluding the pilot channels defined in the other three sets.
Since all PN offsets in a network are time shifted versions of each other, it follows that with appropriate time delay, an incorrect pilot channel from any sector may be mistaken for a pilot channel in the Active Set. However, a large time delay between a mobile unit and a base station implies a large path loss and hence a weak pilot channel signal at the mobile unit. Thus, if the PN offsets of different sectors have a large separation between them, a pilot channel signal would have a very high path loss and hence a very trivial probability of appearing within an active search window of another pilot channel. This makes an appropriate assignment of PN offsets crucial to ensure that a wrong pilot channel would be sufficiently weak so as not to cause any problems.
Reusing PN offsets is possible if: (1) a mobile unit being served by a base station is not interfered with by the pilot channel of another CDMA base station using the same PN offset, or (2) a base station in the network can uniquely identify all the pilot channel signals being reported by a mobile unit that it is serving.
The mobile unit uses a network-selected PILOT.sub.-- INC parameter for the base station to determine which pilot channels to scan from among the Remaining Set, which is the set of all possible pilot channels in the system that are integer multiples of the PILOT.sub.-- INC parameter on the current CDMA frequency assignment, excluding pilot channels in the other sets. A Remaining Set pilot channel is assigned a lower priority in the scanning order, than an Active, Candidate or a Neighbor Set pilot channel.
The setting of the PILOT.sub.-- INC parameter by the network is crucial to PN offset index planning. It has an impact on the mobile unit's Remaining Set pilot channel scanning rate, the amount of co-offset and adjacent offset protection available in the network, and the total number of offsets available. The PILOT.sub.-- INC parameter is an integer having a-valid range from 1 to 15. Low values of PILOT.sub.-- INC provide good co-offset protection, more offsets from which to choose, and less reuse of offsets, and increase the time to scan the pilot channels in the Remaining Set of the mobile unit. High values of PILOT.sub.-- INC, on the other hand, provide good adjacent-offset protection, fewer offsets from which to choose, and more reuse of offsets, and decrease the time to scan the pilot channels in the Remaining Set of the mobile unit.
Conventional PN offset planning methods are based on idealized hexagonal grid structures, and accomplished by fitting a highly irregular pattern of cellular base station locations to a tessellated hexagonal grid pattern. PN offsets are assigned by reusing the same PN offset a specified predetermined number of base stations away.
In order to plan for growth, these methods usually group PN offsets into a few groups, typically three for tri-sectored sites in the network. One sector of a base station is assigned a PN offset from one of the three groups. The other two sectors of the base station are assigned PN offsets from the other two groups, respectively. A few PN offsets in each group are reserved for growth of the network when new base stations are added. The remaining PN offsets in each group are used to make assignments. Such assignments by groups are not optimal in terms of reducing inter-sector interference. Moreover, the unused PN offsets constitute a wasted resource until the network grows.
Such methods make initial assignment choices very easy because the assignments can be made without computer assistance or optimized planning. After an initial assignment, irregularities are accounted for by manual modification of the assignment by an experienced engineer with local knowledge of the environment. Unfortunately, PN offset planning under these idealized assumptions creates many inefficiencies due to the initial assignment's inaccurate reflection of reality.
The irregularities that produce these inefficiencies are due to several factors. First, the need for base stations in a particular area is highly non-uniform because people do not tend to distribute themselves uniformly over large areas. They tend, for example, to cluster in neighborhoods, at work, and in cities. Second, choices for new base station locations are very limited due to factors such as zoning. Base station locations cannot be chosen in ideal locations even if the user traffic was uniformly distributed over a geographic area. Lastly, areas that are covered by base stations are highly dependent upon the propagation environment. Irregularities such as terrain, morphology, and reflecting structures produce highly irregular areas of coverage.
Automatic PN offset planning has heretofore found only limited application due to the need to account for several sets of constraints in a timely manner. Optimization systems must be able to handle large cellular networks with constraints on both co-offset and adjacent offset assignments. For example, the same PN offset cannot be used by neighbors of a base station or neighbors of neighbors of a base station. Adjacent offset protection must be provided to ensure that an adjacent offset does not propagate into a coverage area with significant power to interfere with the pilot channel. Both allowable PN offset separation and interference limits are constraints needed to address this problem adequately.
Many conventional systems handle optimization problems under minimum allowable PN offset separation constraints only. These systems are overly constraining since a range of PN offset prohibitions is needed between base station sectors rather than simply prohibiting use of every offset below a certain threshold interference. These systems also ignore factors such as interference from more than two assignments of the same offset.
By constraining the problem with minimum allowed offset separation between sectors, these systems tend to over constrain the PN offset assignment problem. For PN offset planning, two sectors interfere if the pilot channels arriving at the mobile unit are in phase with each other to within the search window of the mobile unit. The systems look at all possible mobile unit locations to find a range of invalid PN offset separations that they use to constrain the problem. The constraint on PN offset separation, however, results in a range of allowable PN offsets rather than a minimum allowed PN offset separation.
Other systems handle interference as well as separation constraints. These systems are designed, however, for the different problem of making analog frequency assignments. Such problems tend to be large and arise from the need to plan for many assignments per analog frequency. These systems must use less complex methods than those that can be used for PN offset planning.
None of the conventional systems provide optimum PN offset assignment because none of these systems considers all possible interference mechanisms in the network, constraints on co-offset and adjacent offset protection, CDMA border and beacon sites, and preassigned PN offsets. Therefore, a need exists to provide optimum PN offset assignments in CDMA cellular networks.